The disclosure relates to an actuating drive for positioning a final control element, in particular a valve member for a technical processing plant, as well as a method for capturing the position or the regulating distance of the final control element. Known actuating drives often feature a safety function by means of which the final control element can be moved to a safety position such as a completely open or completely closed position of a valve member. In order to realize a safety function, the known actuating drive has at least one return spring.
In many areas of application, the actuating drives are driven by a working fluid, for example hydraulically or pneumatically. These so-called fluid drives are employed in plant automation and process engineering. The fluid drives have a piston or a membrane as a pressure bearing surface. Depending on the mechanical coupling of the piston or the membrane to motion transmitting drives, a linear or pivoting movement can be produced. In the case of failure of the fluidic drive energy, the defined safety position is to be adopted, which is realized by the mechanical return spring.
In many applications, these kinds of actuating drives are part of a control circuit, wherein a position measuring unit detects the position of the final control element. Preferably a fluid is used for powering the drive, but electrical actuating drives realizing a safety position by means of a spring are also known.
A magnetic actuating drive with a spring for the safety position is described by US 20080204175 A1. This application discloses a plunger coil as an electro-magnetic drive which is part of a control circuit, wherein the position of the drive is detected at the driving rod by means of a lever.
EP 917675 B1 describes a positioner for a pneumatic drive with three springs, a position controller and a position reporter coupled to a pushing rod similar to the actuating drive of US 20080204175. A minimum positioning step width is determined and further positioning steps are determined.
Another kind of position detection at a piston drive is described by EP 1752665 B1. Here a pneumatic drive with a microwave position measurement is provided. In this case, there are no springs for the safety position.
EP 1846681 B1 describes an apparatus for controlling a switching movement of a valve. A valve drive in the form of a linear piston with a spring is coupled to a valve member, and the valve movement is measured at the side facing away from the valve by means of position indicators.
Another type of actuating drive is presented in EP 1 593 893 A1 for a safety valve. A piston performs a linear movement that, however, is transformed into a pivoting movement of the shaft. A spring is disposed not directly at the piston but at the opposite side of a driving rod.
A disadvantage of the actuating drives with a safety position available to date is the lack of protection against injury and the mechanical vulnerability against damage when for example a lever or gear mechanism is connected to a drive rod. Besides, the known systems suffer from a lack of flexibility in combining different regulating distances for an actuating drive, in particular if a position measurement is to be provided near the actuating drive.